This invention relates to a method and apparatus for processing a used radioactive waste resin (ion exchange resin) generated in a nuclear power station or the like. More particularly, the present invention relates to a method and apparatus for reducing the volume of the waste resin by pyrolysis and for processing the resin into stable inorganic compounds.
A waste liquor containing a variety of radioactive substances is generated in the course of the operation of a nuclear power station or the like, and the waste liquor is mostly processed using ion exchange resins. The processing of the used radioactive waste resins generated in this instance is one of the problems to be solved for the operation of the nuclear power station. In a power station using boiling water reactors, for example, the used ion exchange resin accounts for the major proportions of the radioactive wastes that are generated.
Conventionally, the used ion exchange resin is mixed with a solidifying agent such as cement or asphalt, is then packed into a drum for solidification and is stored in a storage site. Since the quantity of these radioactive wastes is ever-increasing, however, it has become a critical problem how to secure the storage site and to ensure the safety during storage. If the used resin is stored for an extended period of time, it will be decomposed and perish because it is an organic matter. When carrying out the solidification treatment of the used resin, therefore, it is extremely important to reduce the volume of the resin as much as possible (volume reduction) and to convert it into stable inorganic matter (inorganic conversion). An acid decomposition method has been proposed in the past as one of the methods of volume reduction and inorganic conversion of the used resin. This method includes a so-called HEDL process (Hanford Engineering Development Laboratory's process). In this process, the waste resin is decomposed by concentrated sulfuric acid (about 97 wt %) and nitric acid (about 60 wt %) at a temperature of between 150.degree. and 300.degree. C. Another acid decomposition method is disclosed in Japanese Patent Laid-Open No. 88500/1978, according to which the waste resin is decomposed by concentrated sulfuric acid and hydrogen peroxide (about 30%). In accordance with these acid decomposition methods, however, a large number of difficulties are yet to be solved such as handling of a highly acidic liquor, corrosion of an apparatus by the concentrated highly acidic solution, solidification techniques of the concentrated liquor that is recovered, and so forth, although they provide a large volume reduction ratio because they decompose the resin and evaporate and concentrate the resulting decomposition liquor.
As an alternative, Japanese Patent Laid-Open No. 1446/1982 proposes a method which avoids the use of a highly acid solution but decomposes the waste resin using hydrogen peroxide in the presence of an iron catalyst. However, the problems of this method are that the processing cost becomes high because it needs a large quantity of hydrogen peroxide which is rather expensive, and decomposition itself of the waste resin is not sufficient so that the resin is likely to remain as the organic matter.
Japanese Patent Laid-Open No. 12400/1982 discloses still another method of volume reduction and inorganic conversion of the waste resin. In this method, the waste resin is burnt in a fluidized bed. In accordance with this method, however, generation of combustion residue and scattering of radioactive substances are great, exhaust gases generated in large quantities must also be processed and part of the residues after combustion of the used resin is likely to be deposited onto the furnace wall of the fluidized bed. For this reason, the combustion efficiency drops in the course of the use of the fluidized bed for an extended period. In other words, the residue deposited on the furnace wall must be removed periodically and this is extremely trouplesome.
The processing of the residue after the volume reduction and inorganic conversion of the waste resin is the common problem to all of the prior art methods described above. In other words, 1 to 20 wt % of decomposition residue per used resin before processing remains even if any of these methods are used, and this residue must be processed into a suitable form in order to store it in a drum or the like.